1. Field of the Invention
The present invention relates in general to electric machines and, in particular, to an adaptive winding system and the corresponding control method thereof for electric machines.
2. Description of the Related Art
Since their invention more than a century ago, electric motors and generators have become an indispensable part of human activities. They are found in almost every application in which controlled motion is required. Despite of their widespread use, electric motors so far have only played a very limited role as the primary mover in vehicular application, even though they do have higher efficiency than internal combustion engines. This is largely due to the deficiency of electric energy storage technologies. Currently, purely electricity-powered vehicles have very limited travel range per charge. The limited power density of current battery systems certainly is the primary reason for the limited travel range. Nevertheless, the lack of electric motors that are efficient over a broad speed range and loading conditions also contributed to the low performance of current electric vehicles. Although future successful implementation of electric motors for transportation purposes will depend critically on the development of advanced battery technology, development of new efficient electric motors and generator obviously will also play a vital role if electric vehicles are to gain widespread acceptance.
Among existing motor designs, permanent magnet (PM) motors are perhaps the most efficient due to the fact that no energy is required to energize the permanent magnets. However, this high efficiency operation can only be achieved in a narrow torque and speed range for conventional PM motors, although they can still be used over a wide torque and speed range at lower efficiency. The characteristics of electric motors and generators depend strongly on the magnetic field intensities of the reacting magnets and the inductances and resistance of the windings of the electromagnets. Therefore motors and generators can be optimized to operate in different operating conditions by adjustment of the magnetic field intensity of the magnets and the inductance and resistance of the windings.
Various methods have been proposed to allow for dynamic adjustment of the PM motors to achieve broader high-efficiency operating range. One obvious method for the adjustment of the reacting magnetic intensity of PM motors is by physical movement of the PM. However, the complexity and high cost of the mechanisms required to physically moving the PMs, and the increased risk of failure associated with these mechanisms make this option unfavorable. More recent methods proposed for field-weakening of PM motor usually involve using an auxiliary winding to cancel the magnetic field of the PM. However, these methods require additional energy to achieve field-weakening control, thus are not energy-efficient.
Another method that is commonly implemented to change the operating characteristics of electric machines is by reconfiguring the windings of the electromagnets in these machines. U.S. Pat. No. 6,847,147 utilizes windings with different wire gauge and number of turns that are energized and de-energized with variable voltages for operation at different speed range. In another U.S. Pat. No. 6,853,107 a BLDC motor with dynamically reconfigurable windings configurations shown in FIGS. 1A-1C is disclosed. However, close examination reveals that the rated power in these configurations is not maintained at a constant level. Rated power of the series configuration in FIGS. 1A and 1B would be restricted to only ¼ and ½ of the rated power of the parallel configuration in FIG. 1C, respectively. This is illustrated by the simulated motor characteristic curves of these three configurations shown in FIG. 2. The torque, efficiency and output power as functions of motor speed are shown respectively in FIG. 2 as solid, dotted and dashed lines respectively. In this figure, curves 211, 221 and 231 are, respectively, torque, efficiency and power curves for the winding configuration in FIG. 1A, curves 212, 222 and 232 are curves for the winding configuration in FIG. 1B, and 213, 223 and 233 are for the winding configuration in FIG. 1C.
In transportation applications, the operation of motors can be characterized broadly by two distinctive phases. The first phase, hereafter referred to as the accelerating phase, involves the acceleration of a heavy load (the vehicle and the load it carries) from stationary state to a desired speed. In this phase, the motors are required to deliver the power that is required to rapidly set the load in motion at the desired speed. This phase typically represents only a small fraction of the total operating time. However, because of the high power required to set the vehicle in motion, it is necessary to maintain the efficiency of the motor at high values through the entire acceleration process. In city traffic conditions with frequent stops, the efficiency of the acceleration phase is even more critical for the overall performance of the vehicles. Since virtually all the existing prime movers (internal combustion and electric motors) achieve high efficiency operation only within a small speed and torque range, this is typically achieved by the implementation of an additional gear or belt transmission device.
In the second phase, or the cruising phase, the motors are used to maintain the load speed in response to changing external loading conditions such as the changing slope of the road, or the changing wind drag due to the changes in wind or vehicle speed. In this phase, the required power is usually a fraction of the maximum rated power of the motor during acceleration. Nevertheless, the efficiency of the motors also need to be maintained at a very high value, since the vehicles may spend most of the operating time in this phase, for example, when traveling on highways,
In cruising phase, the maintenance of high efficiency operation when large variation in the external loading condition occurs is typically achieved by the use of the mechanical transmission devices. To compensate small variations in the external loading, the power input to the primary movers can be regulated. In the case of internal combustion engines, constant speed is usually achieved through the throttle control of the fuel injection system. More recent advanced designs can regulate the output power of the engines through the control of the number of active cylinders. In situations when electric motors are used as the prime movers, Pulse Width Modulation (PWM) is a well-established method for regulating the speed of the motors.
The current energy crisis has also spurred renewed interested in environmentally friendly power generation methods that do not rely on the burning of fossil fuel or coals, such as solar and wind power generation. Wind power generation is challenging since it depends on an uncontrollable power source. Wind turbine generators designed for certain wind speed will not be able to operate when the wind speed drops below its designed value. Therefore, it is desirable to have generators that can be dynamically reconfigured to operate over a broad range of wind speed. In modern electric and hybrid vehicles, regenerative braking system has become a critical component of the vehicle to help extending the driving range. Reconfigurable, adaptive generators that can adjust their output current and voltage dynamically according to the charging state of the battery and the driving conditions are obviously very desirable.